1. Field of the Invention
This invention relates to medical imaging systems using nuclear magnetic resonance. In a particular application the invention relates to correcting the magnetic inhomogeneities which vary due to drift or environmental changes.
2. Description of Prior Art
Nuclear magnetic resonance, abbreviated NMR, represents a new approach to medical imaging. For general descriptions of the various imaging methods we can use a number of references including the books, NMR Imaging in Biomedicine by P. Mansfield and P. G. Morris, Academic Press and Nuclear Magnetic Resonance Imaging in Medicine, published in 1981 by Igaku-Shoin, Ltd. Tokyo or review papers including "NMR Imaging Techniques and Applications: A Review," by P. A. Bottomley, Rev. Sci. Instrum., vol. 53, September 1982, pp.1319-1337, and "Fourier Transform Nuclear Magnetic Resonance Tomographic Imaging," by Z. H. Cho, et al., Proceedings of the IEEE, vol. 70, October 1982, pp. 1152-1173. In these, various NMR imaging systems are described, each requiring a highly stable magnetic field because of the critical nature of the magnetic resonance. Subtle variations in these fields can cause either severe distortions or complete loss of the image. As a result of these severe stability requirements, most manufacturers have begun to use superconducting magnets, which are both expensive and difficult to maintain. They are also incapable of reasonably rapid field variations, which is desirable for some studies such as relaxation times as a function of frequency.
This problem of a severe stability requirement was addressed in UK Application GB No. 2076542A by Godfrey N. Hounsfield of EMI Limited. In this patent small pickup coils are used to sample the received signals on either side of the object being studied. The output from these coils form the demodulating signals for the signals received from the principle receiver coil. Thus magnetic field variations during the time signals are received are compensated for.
This approach, however, does not compensate for field variations which cause errors in the transmitter or excitation signals. Also, the method shown requires complex handling of the signals when projections are taken in different directions. At least four separate pickup coils are required, preferably a pair for each projection angle, or a single pair of pickup coils which are rotated for each projection angle.
The problem of errors in the excitation signals was solved in a U.S. patent application Ser. No. 476,474 by the same inventor filed Mar. 18, 1983. Here, following a first excitation signal, the frequency of a reference signal is estimated and used to determine the frequency of a second excitation signal. This insures that the correct plane is excited, independent of drift in the field.
These methods, however, do not compensate for inhomogeneities in the magnetic field which can cause both distortions and artifacts. These inhomogeneities are usually "shimmed" or compensated for using various electromagnetic coils and metallic structures. However, this skimming operation cannot account for changes in the inhomogeneity due to drifts in the field or changes in the environment. Environmental changes are usually avoided by planning the NMR imaging instrument in a larger region where the environment can be carefully controlled. This, however, represents a substantial cost in the inefficient use of space.